Many different types of medical devices are implanted within patients to provide medical therapy. One type of an implanted medical device is a cardiac rhythm management device, such as a pacemaker or implantable defibrillator. Cardiac rhythm management (CRM) devices are used to provide medical therapy to patients who have a disorder related to cardiac rhythm, such as bradycardia or tachycardia.
Magnetic resonance imaging (MRI) is a method of visualizing body tissues of a patient, primarily to identify pathological conditions or to visualize physiological structure for purposes of medical diagnosis and therapy. MRI relies on subjecting the body tissue of interest to a very strong uniform magnetic field, up to about 30,000 gauss, as well as a moderate strength but variable magnetic field of around 200 gauss. In the presence of these uniform and gradient magnetic fields, a radio frequency (RF) pulse is transmitted from a coil to the body tissue. Hydrogen atoms within the body tissue have a magnetic moment and tend to line up with the direction of the applied magnetic fields. Some of these hydrogen atoms will align facing one direction and others will align facing an opposite direction, such that most of the hydrogen atoms facing in alternating directions will tend to cancel each other out. However, a small percentage (but a significant absolute number) of hydrogen atoms will be unbalanced, or not cancelled out. The applied RF pulse tends to cause the unbalanced hydrogen protons to spin, or resonate, in a particular direction and at a particular frequency. When this RF pulse is turned off, the spinning hydrogen protons revert to their earlier, aligned position, and release their excess energy. The RF coil of the MRI machine is capable of detecting this emitted energy and transmitting a corresponding signal to a processor that transforms the signal to an image of the body tissue. Because different tissues have different characteristic responses to the application of the RF pulse in the presence of the magnetic fields, these differences can be utilized to prepare an image showing areas of contrasting tissue types.
MRI techniques have proven to be very effective at diagnosing certain medical conditions and allowing for patients to receive timely, appropriate medical therapy. However, in many cases patients having an implanted medical device are contraindicated for MRI and MRI may be inappropriate for those patients.
During imaging, the electromagnetic radiation produced by the MRI system may interfere with the operation of an IMD. In the case of cardiac devices, the presence of strong magnetic fields and RF energy during an MRI scan may prevent the charging of a high voltage capacitor within the cardiac device, which can affect the ability of the cardiac device to deliver electrical shocks to the patient when an event such as a tachyarrhythmia occurs. The RF energy and/or time varying gradient fields may prevent the sensing and detection of tachyarrhythmias by cardiac devices. The MRI system may also affect other types of implantable medical devices (IMDs).
Some embodiments of IMDs provide for a method of operating an IMD in the presence of an MRI environment using particular MRI settings for the IMD. Examples of patents and patent publications that describe such methods and options for MRI settings for an IMD include U.S. Published Patent Application No. 2010-0087892A1, Stubbs et al.; U.S. Published Patent Application No. 2010-0106215A1, Stubbs et al.; U.S. Published Patent Application No. 2010-0211123A1, Stubbs et al.; U.S. Published Patent Application No. 2008-0221638A1, Wedan et al.; U.S. Pat. No. 7,853,318, Wedan and U.S. Pat. No. 7,873,412, Wedan; which are each incorporated by referenced herein in its entirety.